Invertebrate biodiversity under hot rocks: habitat use by the fauna of sandstone outcrops in the Sydney region

Loose surface rocks on sandstone outcrops in coastal eastern Australia are subject to high levels of anthropogenic disturbance. The endangered reptiles that use these rocks as shelter-sites have attracted considerable research, but the invertebrate fauna under the same rocks has been neglected. We surveyed four sites in the Sydney region, to record the invertebrate fauna under a total of 240 rocks and quantify habitat associations of these animals. Fauna were present under almost 90% of surveyed rocks, with high overall abundance and species richness (up to seven species per rock). Our sampling regime was inadequate to fully characterise the sandstone invertebrate fauna, and thus substantially underestimates diversity on a broader spatial scale. The taxa covered a broad phylogenetic range, with ants and spiders the most abundant, widespread and species-rich lineages. The assemblage was dominated by predators and opportunistic scavengers, possibly reflecting the characteristic low productivity of these systems. Canonical correspondence analysis identified consistent linkages between specific faunal groups and microhabitat features (mainly substrate, also degree of shading and characteristics of the overlying rock). We conclude that surface rocks on Hawkesbury sandstone outcrops shelter a diverse and abundant invertebrate fauna, that microhabitat features influence faunal composition, and that our ignorance of this component of biodiversity will severely hamper any rational plan to conserve these distinctive and highly threatened ecosystems.